Molecular Shapes

Trigonal Planar – AX3

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The formula is AX3 and an example would be SO3

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It tells us there are three bonds to the central atom

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We start with the Lewis structure for SO3

There are 4 BP and 0 LP of e-
around the central atom!

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These central atom e- tell us SO3has a trigonal planar geometry
with bond angles of 120⁰

It is 120⁰
(1/3 of a circle)
from bond to bond

S

O

O

O

Practice Problem

What geometry and bond angle would CBr4 have?

CBr4

There are 32 total valence e-:
C: (1 atom)(4 valence e-) = 4 e-

Br: (4 atoms)(7 valence e-) = 28 e-

C

There is only one carbon atom, so it is our central atom!

Fill in bonding e- first!

Fill the outermost atoms next

All of the e- have
been used and all of
the atoms are
happy with 8 e-

Br

Notice there are 4 BP and 0 LP of e- around the central atom!

Br

4 BP & 0 LP tells us it is:

tetrahedral w/ bond
angles of 109.5⁰

Br

Br